The development of prolactin cell hyperplasia and the transformation to neoplastic prolactin producing pituitary tumors will be investigated in rats using morphological, immunohistochemical and biochemical methods. Morphological and immunohistochemical studies at the light and electron microscopic levels will be used to characterize various subtypes of prolactin cells and mammosomatotropic cells (which produced both prolactin and growth hormone) in hyperplastic and neoplastic MtT/W15 and MtT/F4 tissues. The functions of the pituitary folliculostellate cells in normal, hyperplastic and neoplastic pituitaries will be investigated by immunohistochemical and ultrastructural studies. The role of S100 protein in regulating prolactin secretion will be investigated. Preliminary findings indicate that S100 protein which is present in pituitary folliculo-stellate cells can stimulate prolactin releases in vitro suggesting that these cells may exert a paracrine type regulation of prolactin release. Enriched populations of neoplastic prolactin, growth hormone and mammosmatotropic and folliculo-stellate cells from hyperplastic pituitaries will be prepared by velocity sedimentation at unit gravity and density gradient centrifugation. These enriched cell populations will be used to study prolactin release in vitro and to analyze dopamine and estrogen receptors in relatively homogeneous cell population. Recent studies have shown that estrogens inhibit the growth of transplantable MtT/W15 and MtT/F4 pituitary tumors and concurrently increases growth hormone production by the MtT/W15 tumor. We plan to use in situ and cytoplasmic dot blot hybridization to analyze and quantitate changes in growth hormone and prolactin messenger RNA in tumor cells during estrogen-induced growth-inhibition. The role of estrogen and dopamine in the regulation of their receptors during development of pituitary tumors will be analyzed by studying changes in receptor levels during transformation from hyperplasia to neoplasia. The goals of this study are 1) to define more clearly the morphological and biochemical features of hyperplastic and neoplastic pituitary tissues, 2) analyze the role of folliculostellate cells and of S100 protein is regulating prolactin cell hyperplasia and regression and the growth of autonomous tumors, 3) analyze the role of estrogen and dopamine and their receptors in pituitary tumor development and 4) develop more refined and specific methods to analyze the regulation of prolactin and growth hormone biosynthesis at the cellular level in hyperplastic and neoplastic pituitaries.